Internet Protocols Fall 2006 Lectures 11-12 Inter-domain routing, mobility support, multicast routing Andreas Terzis
Outline Inter-domain Internet Routing BGP Routing for mobile nodes Multicast routing CS 349/Fall06 2
Internet Routing Hierarchy large ISP EGP large ISP medium ISP dialup ISP dedicated access ISP AS (Autonomous System) - a collection of routers under the same technical and administrative domain. EGP (External Gateway Protocol) - used between two AS s to allow them to exchange routing information so that traffic can be forwarded across AS borders. Example: BGP CS 349/Fall06 3
Questions How do we tie the two routing levels together? What are the special constraints of inter-domain routing? What is the routing algorithm used at the inter-domain level? CS 349/Fall06 4
Two-Tier Routing I-BGP R2 R3 AS1 E-BGP AS2 A announce B R1 R border router R4 AS3 R5 internal router CS 349/Fall06 5 B
So Many Choices AS 4 Hopkins AS 3 AS 2 Which route should Hopkins pick to 13.13.0.0./16? AS 1 13.13.0.0/16 CS 349/Fall06 6
Choice of Routing Algorithm Constraints: scaling Support policy-based route selection Link-state? requires sharing of complete network informatin information exchanges doesn t scale can t express policy Distance Vector? scales and retains privacy can t implement policy can t avoid loops if shortest paths not taken CS 349/Fall06 7
Path Vector Protocol Distance vector algorithm with extra information For each route, store the complete path (ASs) No extra computation, just extra storage Advantages: Can make policy choices based on set of ASs in path Can easily avoid loops CS 349/Fall06 8
BGP Operations (Simplified) Establish session on TCP port 179 AS1 BGP session Exchange all active routes AS2 Exchange incremental updates While connection is ALIVE exchange route UPDATE messages CS 349/Fall06 9
Four Types of BGP Messages Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. Announcement = prefix + attributes values CS 349/Fall06 10
BGP Attributes Value Code Reference ----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen]... 255 reserved for development CS 349/Fall06 11
Attributes are Used to Select Best Routes 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route (Note: it could reject them all!) CS 349/Fall06 12
BGP Next Hop Attribute 12.125.133.90 AS 6431 AT&T Research AS 7018 AT&T 12.127.0.121 AS 12654 RIPE NCC RIS project 135.207.0.0/16 Next Hop = 12.125.133.90 135.207.0.0/16 Next Hop = 12.127.0.121 Every time a route announcement crosses an AS boundary, the Next Hop attribute is changed to the IP address of the border router that announced the route. CS 349/Fall06 13
ASPATH Attribute 135.207.0.0/16 AS Path = 1755 1239 7018 6341 AS 1129 Global Access 135.207.0.0/16 AS Path = 1239 7018 6341 AS 1239 Sprint AS 1755 Ebone 135.207.0.0/16 AS Path = 7018 6341 135.207.0.0/16 AS Path = 1129 1755 1239 7018 6341 AS 12654 RIPE NCC RIS project 135.207.0.0/16 AS Path = 6341 AS 6341 AT&T Research 135.207.0.0/16 Prefix Originated AS7018 AT&T 135.207.0.0/16 AS Path = 7018 6341 135.207.0.0/16 AS Path = 3549 7018 6341 AS 3549 Global Crossing CS 349/Fall06 14
Interdomain Loop Prevention BGP at AS YYY will never accept a route with ASPATH containing YYY AS 7018 Don t Accept! 12.22.0.0/16 ASPATH = 1 333 7018 877 AS 1 CS 349/Fall06 15
BGP Route Processing Receive BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values Best Routes Apply Policy = filter routes & tweak attributes Transmit BGP Updates Apply Import Policies Best Route Selection Best Route Table Apply Export Policies Install forwarding Entries for best Routes. IP Forwarding Table CS 349/Fall06 16
Route Selection Summary Highest Local Preference Enforce relationships Shortest ASPATH Lowest MED i-bgp < e-bgp Lowest IGP cost to BGP egress Lowest router ID traffic engineering Throw up hands and break ties CS 349/Fall06 17
Local Preference AS 4 local pref = 80 local pref = 90 AS 3 local pref = 100 Higher Local preference values are more preferred AS 2 AS 1 13.13.0.0/16 CS 349/Fall06 18
Shortest AS Path Route Selection Algorithm 1. Local Pref 2. Shortest AS Path Shortest AS Path!= shortest path BGP says that path 4 1 is better than path 3 2 1 AS 3 AS 4 AS 2 AS 1 CS 349/Fall06 19
Outline Inter-domain Internet Routing BGP Routing for mobile nodes Multicast routing CS 349/Fall06 20
Routing for Mobile Hosts Internet routing does not support mobile hosts IP address is divided into network part and host part Rest of the Internet knows how to forward packets to the network end-host is connected to If end-host moves packets are delivered in the wrong network CS 349/Fall06 21
Partial Solution As mobile node moves it acquires different IP addresses DHCP This is adequate for some applications Web browsing, etc Does not work for other applications Mobile host is the server Long lasting connections CS 349/Fall06 22
Mobile IP Solution proposed to solve the problem of mobility Does not require changes in most IP infrastructure Introduces three new entities Home Agent (HA) Foreign Agent (FA) Mobile Host (MH) CS 349/Fall06 23
Example Sending host Home agent (10.0.0.3) Internetwork Foreign agent (12.0.0.6) Home network (network 10) Mobile host (10.0.0.9) CS 349/Fall06 24
Sub-problems How does the home agent intercept packets that are destined to the mobile node? How does the home agent deliver the packet to the foreign agent How does the foreign agent deliver the packet to the mobile node? CS 349/Fall06 25
Home Agent Intercept Packets from Corresponding Agents (CAs) arrive at the home network Home agent issues proxy ARP Replies with its own MAC to queries for the Mobile Hosts IP Packets are delivered to the Home Agent When Mobile Host returns to the home network Gratuitous ARP CS 349/Fall06 26
Tunneling A tunnel exists between HA and FA Home agent encapsulates IP packets for the Mobile Host using the FA s IP address Rest of the Internet forwards packet as if it was destined for the FA When packet arrives at the FA, it decapsulates the packet CS 349/Fall06 27
FA Delivery FA bypasses regular IP routing for registered MHs and delivers directly to the MAC address MH can be the FA What about packets sourced by the MH? Sent from the remote network with the MH s permanent address Reverse tunnel CS 349/Fall06 28
Route Optimization Packets from CA ->HA->FA follow suboptimal route If CA is upgraded it can create tunnel directly to the FA HA sends binding update to the CA when packets arrive to it FA can also send binding updates Security implications? CS 349/Fall06 29
Outline Inter-domain Internet Routing BGP Routing for mobile nodes Multicast routing CS 349/Fall06 30
Multicast Routing Applications that want to send same data to multiple destinations Audio, Video, Stock Updates Multicast model Sender sends to Multicast Address Receivers join Multicast Address IGMP Packets are delivered to all hosts that joined the multicast address How do the receivers know which multicast group to join? CS 349/Fall06 31
Distance-Vector Multicast DVMRP uses a two step approach Flooding mechanism that delivers packets to the whole network Pruning networks that have no receivers CS 349/Fall06 32
Reverse-Path Broadcast Flood packets from S to everyone Router forwards packets to all interfaces (minus incoming) if packet arrives on correct interface Interface R would use to send packets to S Packet will be sent multiple times if multiple routers connected on same LAN Designate parent for each network Router with shortest distance to S A B R1 R2 R3 R4 R5 R6 C R7 CS 349/Fall06 33
Reverse-Path Multicast RPB delivers packets to all networks Obviously undesirable Leaf networks know if any interested receivers are present Routers propagate this information to upstream neighbors Tree is pruned What happens if a new receiver joins at a pruned network? CS 349/Fall06 34
Alternatives DVMRP not appropriate for sparse groups Protocol Independent Multicast (PIM) solves this issue Rendezvous Point (RP) CS 349/Fall06 35